Membrane bioreactors, which use biological matter in conjunction with a membrane to convert materials supplied to the biological matter, may be used for bioreaction applications such as the production of useful substances, for example pharmaceuticals, antibodies or vaccine components, the bioconversion of organic wastes into biomass or biofuels, or remediation of toxic wastes including degradation of toxic chemicals to inert or non-bioavailable forms and precipitation or reduction/oxidation of heavy metals.
Broadly speaking, existing bioreactors can be classified as mechanically agitated bioreactors, pneumatically agitated bioreactors or non-agitated bioreactors. Mechanically agitated bioreactors include: aeration-agitation bioreactors; rotating drum bioreactors; and spin-filter bioreactors. Pneumatically agitated bioreactors include sparge bioreactors, and air-lift bioreactors. Non-agitated bioreactors include gaseous phase bioreactors, oxygen-permeable membrane aerator bioreactors, and overlay aeration bioreactors.
Pneumatically agitated bioreactors typically consist of a vat fitted with aeration vents that sparge air through the contained liquid medium, to maintain an adequate supply of dissolved oxygen for the biomass. Such reactors use a variety of systems to ensure that the biomass and process liquor remain well mixed, including impellers, propellers, and paddles. Paddles are also used to scrape biomass from the sides of the vessel to minimise fouling and ensure that the biomass remains in contact with the process liquor. However a disadvantage with such systems is that the shear forces associated with such mixing and scraping can often damage fragile cultures, leading to a reduction in biological activity and a consequent reduction in productivity. As well, the presence of the biomass, which is relatively dense, increases the viscosity of the reaction medium, thus reducing both mixing efficiency and the rate of diffusion of molecular oxygen and other gases within the process stream. Any reduction in the availability of oxygen leads to a corresponding reduction in the activity of the biomass, ensuring that many cell types no longer function as they would in natural systems (e.g. at the air-solid interface or, in the case of animal cells, while bathed in blood).
Tissue culture systems include sparged bioreactors and a variety of submerged surface-growth systems in culture vessels or rolling drums. A disadvantage of these systems is that the uptake of oxygen is relatively low, and hence the bioavailability of dissolved oxygen becomes limiting once small amounts of biomass have grown. The low availability of dissolved oxygen prevents many types of cells from being cultured, and many cell lines do not function as they would in the body, where oxygen is more readily available.
In packed column systems, cells are immobilised on inert materials of various shapes such as rings, spheres saddles or polygons which are packed into a column. A nutrient stream is oxygenated prior to being fed to the column. A disadvantage of these systems is that they are limited by the solubility of oxygen in the nutrient stream. They are usually run in trickling mode and oxygen limitation may also relate to thickness of the biomass. A further disadvantage is that growth of the cells can lead to agglomeration of the packing and to clogging of the column. Cost is also an issue for highly engineered versions of these.
Secondary sewage treatment may involve activated sludge bioreactors coupled to a clarification system. The biomass in the activated sludge is high (typically 3000 mg/L) and is oxygenated by sparging air. The dissolved oxygen concentration is typically 0.5 mg/L and the biomass oxidises the soluble reduced organic molecules, the ammonium cations to nitrate anions, and also flocculates particulate and suspended cellular matter so that it settles easily in the clarification system downstream. The clarification system separates the majority of the suspended solids from the liquid component using sedimentation and recycles the solids/biomass back to the activated sludge bioreactors to maintain the high biomass concentration needed to conduct these processes. The process of sparging the activated sludge consumes a lot of energy.
Previous membrane based systems for treating sewage have used immersed hollow fibre membranes. In these systems, the membrane is immersed into the sewage liquid to be treated, and gas is passed from the lumen of the membrane through the membrane itself and provides oxygen to a biofilm located on the outside of the membrane. The membranes require the strength to withstand a pressure of gas necessary to penetrate the membrane, and provide relatively inefficient bioremediation of the sewage liquid. Another system for sewage treatment is the trickle bed system. In this system, the treatment bed comprises relatively large particles provided with a biolayer capable of treating the sewage. Air is sparged upwards through the bed and sewage is passed down through the bed. As the sewage is contacted with the air and biolayer, it is treated by the biolayer, and treated sewage exits through the lower regions of the bed. This system is relatively inefficient, and is prone to fouling
There is therefore a need for an efficient and robust system for removing organic matter and ammonium ions from sewage. A suitable system may operate by effectively retaining biomass and providing a cheap and effective aeration system.